Tailing Impoundment, Mine Pit and Pit-Lake Example
Description
The Tailing Storage Facility, Mine Pit, and Pit-Lake Example is designed to show most of the FFTools Modules working together in a single model:
- CRB is used to specify stream stages at one end of the model,
- SES controls the activation of elements representing a tailings storage facility (TSF) and inactivation of elements in a mine pit,
- SHB controls fixed-head nodes related to the tailings impoundment and the dewatering of the mine-pit,
- LAKE3D models pit-lake formation after the end of mining, and
- OBS is used to record simulated water levels at user-selected nodes which are specified using a setup file in CSV format.
The model has 52 layers of variable thickness. There are 2392 nodes per slice and 2390 elements per layer. The dimensions of every element is the same. This is a 3D model of unit width (most FFTools modules are designed to work with 3D models using the layered prism geometry). There are three primary hydrologic units:
- Alluvium (basin fill and a small creek channel extending from the valley fill up hill to be eventually buried under the TSF (K = 5 m/d),
- Bedrock and ore (K = 0.000864 to 0.00864 m/d), and
- Tailings (K = 0.00173 to 0.173 m/d).
The TSF is composed of three zones: The embankment consists of coarser materials which grades to a zone of material of intermediate grain size, which grades into a zone of fines. Elements representing the TSF (in the top 25 layers of the model) are inactive at the start of the model run.
Recharge varies across the model, ranging from zero over the basin fill up to 6.95e-4 (m/d) at the mountain top. The initial condition for the transient model is based on steady-state flow under pre-mine recharge and other boundary conditions. As the TSF grows (36 m/yr), TSF elements are activated at full saturation and a decant pond is simulated next to where the impoundment laps onto the face of the mountain. Fixed-head nodes simulate the active top of the TSF (a wet beach) using constrained nodes that allow seepage of water into the impoundment only; the fixed head at each of these "TSF nodes" is set equal to the elevation of the active top of the structure. As the TSF grows, the TSF nodes are removed when buried. When mining is complete, tailings discahrge to the TSF ceases and these fixed heads are removed (dry beach). Drains are simulated under the TSF using constrained fixed-head nodes (water can be removed, but not added to model) using SHB.
The mine is excavated at a constant rate of 76.5 m/yr. When an element is completely excavated it is inactivated by SES according to the SES setup. Any groundwater in the excavated element is accounted for in the SES budget, but is reflected in FEFELOW's water budget. As the floor of the pit is lowered, evaporation and sumps remove discharging groundwater; this is simulated using SHB to set fixed-head nodes on the wall and floor of the pit, "Pit Nodes". The Pit Nodes allow only discharge of groundwater.
At the end of mining and discharge to the TSF (10 yrs; 3650 days), SHB ceases its work (all associated fixed-head nodes are removed), and the LAKE3D module begins operation to simulate formation of a pit lake in the former mine pit. The TSF is allowed to drain freely. LAKE3D sets boundary conditions transiently as the pit-lake grows to simuate groundwater discharge (or recharge) from the walls and floor of the pit. This module also records flows to and from the lake, the lake stage and volume, and other relevant information. The model simulates 90 years of pit lake flows and TSF drain down (total simulation time is 100 years or 36,500 days). Precipitation on to the walls of the pit and pit-lake surface is 0.0069 m/day, groundwater recharge on undersaturated pit-wall elements is 0.00069 m/day, evaporation from the pit lake surface is -0.008 m/dy, and surface run-off into the lake is 0.0014 m/day.
- Hydraulic conductivity distribution and initial (pre-mining) water levels.
- SES elements: TSF elements in red and mine-pit elements in brown.
- Fixed-head nodes representing pre-mine streams and, later, drains under the TSF; the water level is set to the elevation of land surface.
FFTools Setup
Following are listings of files imported into the various FFTools modules to set up the FEFLOW model (also supplied, see below). XML files are located in the import+export directory (not listed here due to length).
CRB setup: Examples/import+export/ex.TSF_Pit_PitLake.crb
CRB
2394,874,0.5,1,250,0
4784,874,0.25,1,250,0
SES setup: Examples/import+export/ex.TSF_Pit_PitLake.ses
SES
GROUP
#1,3,0,1
XML,[ModelIEPath]ex.TSF_Pit_PitLake.Pit_ElemSet.xml
0,3650,1695,930
#2,3,1,1,0.000000,6,1.000000
XML,[ModelIEPath]ex.TSF_Pit_PitLake.TSF_ElemSet.xml
0,3650,970,1330
SHB setup: Examples/import+export/ex.TSF_Pit_PitLake.shb
SHB
GROUP
! Mining Pit
#1,1,0,T,0.1,2
XML,[ModelIEPath]ex.TSF_Pit_PitLake.Pit_NodeSet.xml
0,3650,1695,930
! Tailings Drainage
#2,-1,0,F,0.05,6
XML,[ModelIEPath]ex.TSF_Pit_PitLake.TSF_NodeSet.xml
0,3650,970,1330
LAKE3D setup: Examples/import+export/ex.TSF_Pit_PitLake.l3d
LAKE,1,3
931,1445.45,0,932.000000
XML,[ModelIEPath]ex.TSF_Pit_PitLake.Pit_ElemSet.xml
3650,36500,0.0069,0.00069,-0.008,0.00138,0
OBS setup: Examples/import+export/ex.TSF_Pit_PitLake.obs_input.obs
NAME, X, Y, TOC_ELEV, TOS_DEPTH, BOS_DEPTH, VALUE
A,5500,0.5,1695,1700,1694,1736.4
B,4749.790795,0.5,1430,1430,1429,1303.4
C,4073.221757,0.5,1298,1300,1297,1246.2
D,2618.828452,0.5,1320,1321,1319,1116.3
E,1173.640167,0.5,973,974,972,941.6
F,5500,0.5,931,932,930,1372.9
G,3062,0.276,1317.7,1188.1,1092.9,-9999
OBS tool help: Observation (OBS) Module.
FEM File
- See examples/femdata/ex.FFTools.TSF_Pit_PitLake.fem
Results
Results from several time periods are shown below (potentiometric contours in black, water table (phreatic surface) shown as a white line, saturation shaded in rainbow colors with red indicating full saturation, fixed-head nodes as light-blue circles (active maximum and minimum constraints indicated with bars above [max] or below [min] the circle), inactive elements in gray):
- 10 Days: mining has just begun
- 5 Years : mid-point of mining
- 10 Years: end of mining and discharge to TSF
- 20 Years: TSF drying out, and the mine pit begins to fill with water. The simulated level, or stage, of the lake, 964 m (listed in the LAKE3D log file), is indicated by an arrow.
- 60 Years: TSF nearly dry, and the pit continues to fill with water. The simulated level, or stage, of the lake, 1,035 m (listed in the LAKE3D log file), is indicated by an arrow.
- 100 Years: TSF essentially dry, and the pit continues to fill with water. The simulated level, or stage, of the lake, 1,065 m (listed in the LAKE3D log file), is indicated by an arrow.
- Lake stage and volume over time